There is an increasing interest in electric light bulbs which do not make use of incandescent filaments, since filament-based light bulbs are considered to be inefficient and energy hungry. Indeed, recent legislative changes mean that traditional incandescent light bulbs are being phased out in many parts of the world. One existing replacement for the incandescent light bulb is the compact fluorescent tube bulb.
Solid state lighting, for example light emitting diode (LED) or organic light emitting diode (OLED) based retrofit lamps, offer superior performance over compact fluorescent lamp (CFL) based retrofit lamps in terms of efficiency, instant light output, light quality, and lifetime. The main barrier to penetrate the market is product cost, since the shop price of today's LED-based lamps can be up to 10 times that of CFL lamps.
A key element of an LED lamp assembly is the LED light source. The luminous efficiency, measured in lumen per watt, has been improved significantly over the last 10 years, and continues to increase further to levels of 250 lm/W for white light LEDs, with potential for further improvement.
Another strong advantage of using LED light sources is that they offer superior lifetime since the only failure mode is a slow depreciation of the outcoupling optics of the light source.
In order to exploit the enormous advantages of LED in terms of its luminous efficiency and its potentially superior lifetime, it is necessary to maintain the LED device strictly below its specified maximum temperature.
Although the LED offers excellent luminous efficiency over alternative light source technologies one strong disadvantage of any solid state light source is that there is no radiation of energy in the form of infrared radiation so that any power losses inside the light source component has to be propagated to the environment purely by heat conduction.
Another aspect of maintaining the LED at or below a desired temperature is that the efficiency of the device degrades with increasing temperature, thereby increasing further the device temperature, assuming the power provided by the power supply is kept at a constant level.
The fast and dynamic improvement of LED efficiency can be utilized only if the manufacturer of solid state light source (SSL) based lamp assemblies can change their product to accommodate new generations of SSL components. This requires normally a redesign of components within the power supply or driver unit inside the SSL lamp assembly due to changing requirements of the SSL. Such redesign tasks are resource and time intensive and increase heavily the total cost for the manufacturer of SSL lamp assemblies.
One major cost factor in manufacturing SSL lamp assemblies are the SSL devices. The manufacturing of SSL devices involves process steps with major statistical variance. Small variations in manufacturing process steps yield shifts in the dominant wavelength that an SSL chip radiates and this impacts directly the luminous characteristic of white light LEDs and OLEDs. Also the manufacturing process impacts the forward voltage of SSL devices which contributes substantially to the power that is delivered to the device when operated with a power supply unit with current source characteristic.
Due to these said large variance of SSL productions such devices are selected according to their respective characteristic into so called bins. Binning adds major complexity to all stages of the supply chain and results in substantial cost premiums.
Another aspect is that the interaction of the SSL device and the power supply unit within the SSL lamp assembly again extends the tolerance window of the actual optical and electrical operating point at which the SSL device is operating in a given application and location.
Accordingly, it is desirable to produce a driver circuit for a solid state light bulb assembly which enables the drawbacks of the existing designs to be overcome.